Composite conductor

ABSTRACT

A composite electrical conductor wherein a sheath of titanium, zirconium or tantalum or an alloy thereof, encasing a metallic core element, e.g. copper and aluminum and bonded thereto and having a coating of &#34;tinning&#34; metal or alloy thereon.

This is a division, of application Ser. No. 395,595 filed Sept. 10, 1973now U.S. Pat. No. 3,991,929, dated Nov. 16, 1976.

This invention relates to a method of coating the inside of a sheath oftitanium, zirconium or tantalum (or an alloy of one of the said metals)with a "tinning" metal or alloy and to a method of bonding the coatedsheath to another metal, especially but not exclusively another metal ofgreater electrical conductivity than titanium, zirconium or tantalum, oralloys thereof.

The invention finds particular application in the production of acomposite comprising a sheath of titanium, zirconium or tantalum (or analloy thereof) bonded to an internal copper conductor.

The presence of an oxide layer on the surface of the sheath of titanium,zirconium or tantalum prevents the surface from being readily wetted bya "tinning" metal or alloy, and thus these metals are not easilysoldered to other metals. It is usually necessary to pre-clean thesurface to remove the oxide, for example by pickling in a strong acid.The pre-cleaned titanium, zirconium or tantalum may then be bonded toanother metal, by, for example, electroplating with copper orelectroless plating with nickel, followed by soldering in a knownmanner.

In the specification of our U.K. Pat. No. 1,236,997 there is describedand claimed a method of coating a sheet, as opposed to a sheath, oftitanium, zirconium or tantalum (or an alloy of one of the said metals)with a "tinning" metal or alloy which avoids the necessity ofprecleaning the surface to be coated. The aforesaid method comprisesheating the said sheet whilst the surface to be coated is covered withthe "tinning" metal of alloy in the molten state and moving anultrasonically excited probe over substantially the whole of the surfaceto be coated, the said probe being in contact with the said surface andwith the molten metal or alloy. We have now surprisingly found that asheath of titanium, zirconium or tantalum (or an alloy thereof) may becoated on its inner surface with a "tinning" metal without having tomove the ultrasonically excited probe over substantially the whole ofthe surface to be coated whilst being in contact with both the innersurface and the molten "tinning" metal.

Thus according to one aspect of the present invention there is provideda method of coating the inner surface of a sheath of titanium, zirconiumor tantalum (or an alloy of one of the said metals) with a "tinning"metal or alloy which comprises heating the said sheath whilst the innersurface is covered with the "tinning" metal or alloy in the molten stateand moving an ultrasonically excited probe over substantially the wholeof the outer surface of the sheath, the said probe being in contact withsaid outer surface. Preferably, the ultrasonically excited probeconforms in shape to part or all of said outer surface.

By the term "tinning" metal or alloy we mean a metal or alloy which willform a coating on the titanium, zirconium or tantalum (or an alloy ofone of the said metals) and which will enable the coated titanium,zirconium or tantalum (or alloy thereof) thus obtained to be used in aconventional soldering process.

By the term "sheath" we mean an article akin to a tube as distinct fromthe planar sheet referred to in our U.K. Patent No. 1,236,997. Thecoated sheath thus obtained may then be bonded to another metal,preferably an electrically conducting metal by a conventional process ofsoldering.

Thus according to another aspect of the present invention there isprovided a method of making a composite by bonding a sheath of titanium,zirconium or tantalum (or an alloy of one of the said metals) to anothermetal, preferably a metal of greater electrical conductivity thantitanium, tantalum or zirconium (or an alloy thereof), which comprisesthe steps of

i. coating the inner surface of the sheath of titanium, zirconium ortantalum (or an alloy of one of the said metals) with a tinning metal oralloy by the method hereinbefore defined and

ii. soldering the coated sheath thus obtained to the other metal.

The sheath of titanium, zirconium or tantalum (or an alloy thereof) may,for example, have a square, rectangular or triangular cross-section, butit is preferred to use sheaths having a circular cross-section.

The preferred electrically conducting metal is copper, although otherelectrically conducting metals may be bonded to the coated titanium,zirconium or tantalum or their alloys, for example aluminium.

It is preferred to use tin itself as the tinning metal. A wide range oftinning alloys may be used. Suitable tinning alloys include binaryalloys containing a major proportion of tin and a minor proportion ofbismuth, cadmium or zinc; binary alloys containing a major proportion ofbismuth and a minor proportion of cadmium, lead, tin or zinc; binaryalloys containing a major proportion of cadmium and a minor proportionof bismuth or zinc; and binary alloys containing a major proportion ofzinc and a minor proportion of bismuth, cadmium, lead or tin. Othersuitable alloys include ternary tin-containing alloys including minorproportions of zinc and lead. It is preferred to use a tin/zinc alloy,especially a tin/zinc alloy containing a minor proportion of zinc.

The electrically conducting metal is preferably pretinned before beingbonded to the titanium, zirconium or tantalum or their alloys. This maybe carried out in the conventional manner by heating the surface to bebonded with an alloy of tin and lead, for example an alloy containing70% by weight of tin and 30% by weight of lead. Alternatively, an alloyof lead and bismuth or other suitable solder may be used.

A wide range of alloys may be used for soldering the coated sheath oftitanium, zirconium or tantalum (or an alloy thereof) to theelectrically conducting metal; suitable soldering alloys include, forexample, alloys of tin and lead and alloys of lead and bismuth. It isespecially advantageous to use alloys of lead and bismuth containingmore than 50% by weight of bismuth, for example 55% bismuth, since thesealloys expand on cooling thereby strengthening the bond.

When the coating process described herein is applied to an alloy oftitanium the alloy may be, in particular, an alloy having polarisationproperties comparable with those of titanium. Examples of such alloysinclude titanium/zirconium alloys containing up to 14% by weight ofzirconium, alloys of titanium with up to 5% by weight of a platinumgroup metal and alloys of titanium with niobium or tantalum containingup to 10% by weight of the alloying constituent.

By a "platinum group metal" is meant one of the metals platinum,rhodium, iridium, ruthenium, osmium and palladium.

The temperature at which the sheath of titanium, zirconium or tantalum(or an alloy thereof) may be coated with the tinning metal or alloy mayvary over a wide range, but it is generally preferred to use atemperature in the range from 350° C. to 450° C., for example from 380°C. to 410° C.

In the case when the sheath is a cylindrical tube the head of theultrasonic probe is concave in shape with a radius of curvatureapproximately equal to the sheath radius. Otherwise, the ultrasonicprobe may be of a conventional design resonating, for example, at afrequency of approximately 20 k/cs.

The coating may conveniently be carried out by enclosing the sheath in avertical furnace adapted to open into two halves, thereby allowingaccess of an ultrasonic probe to the outer surface of the sheath. Thesheath is stoppered at one end, filled with the tinning metal or alloyand then inserted into the furnace. The sheath and its contents areheated until the tinning metal or alloy is melted and the furnace isthen partly opened. The ultrasonic probe is moved up and down the outersurface of the sheath whilst keeping the probe in contact with thesurface. At the same time, the sheath is rotated so that substantiallythe whole of the outer surface of the sheath comes into contact with theprobe. The sheath is then removed from the furnace and emptied of metalor alloy.

The bonding of the coated sheath of titanium, zirconium or tantalum ortheir alloys to another metal, preferably pretinned electricallyconducting metal, may conveniently be carried out by inserting a closelyfitting rod of the conducting metal into the coated sheath and melting alead/tin or a lead/bismuth alloy above the rod whilst maintaining thesheath in a vertical position.

The invention is especially advantageous in the production oftitanium/copper or titanium/aluminium conductors having a low electricalresistance at the bonded surfaces. The titanium/copper conductors areparticularly useful in electrolytic diaphragm cells and chlorate cells;thus one or more titanium/copper conductors may be successfullyspot-welded to a titanium anode plate without adversely affecting theelectrical properties of the titanium-copper bond.

The invention is illustrated, but not limited, by the followingExamples.

EXAMPLE 1

A titanium tube (3/4 inch external diameter; 20 gauge thickness) wassealed at one end with a mild steel plug, filled with a tin/zinc alloy(75% tin, 25% zinc) and inserted into a vertical furnace. The furnacecomprised two separate halves which were adapted to swing between afully closed position (thereby completely surrounding the tube) and apartly or fully opened position. The tube and its contents were heatedto 400° C. with the furnace in its fully closed position. The furnacewas then partly opened. An ultrasonically excited probe, consisting of ahalf wave-length resonant steel probe (having a natural frequency of 20k/cs) coupled to a magnetostriction transducer, and having a concavehead (approximately 3/8 inch radius of curvature), was moved up and downthe outer surface of the tube whilst keeping the probe in contact withthe surface. At the same time the tube was rotated until substantiallythe whole of the outer surface of the tube had come into contact withthe tube. The tube was then removed from the furnace and the surplusalloy drained from the tube.

The electrically conducting metal for insertion into the coated titaniumtube consisted of a copper rod (5/8 inch diameter) which had a slightlyincreased diameter at one end, and which was also provided with aterminal connection at this end. The copper rod was pre-tinned with alayer of tin/lead alloy (70% tin, 30% lead) at 120°- 130° C.

The pre-tinned copper rod was pushed into the coated titanium tube, theincreased diameter of the rod at the terminal and effectively providinga seal. The tube containing the rod was then introduced into thevertical furnace with the terminal at the lower end of the tube, andlead/bismuth alloy (45% lead, 55% bismuth) was introduced into the upperpart of the tube. The tube was heated to 150° C. and maintained at thistemperature for 5 minutes. The tube containing the rod was then takenout of the furnace and allowed to cool.

EXAMPLE 2

The titanium tube was coated with a tin/zinc alloy (75% tin, 25% zinc)using the procedure described in Example 1.

Two copper rods (each 5/8 inch diameter) were pretinned with a layer oftin/lead alloy (70% tin/30% lead) at 220° - 230° C.

One of the pre-tinned copper rods was inserted into the tube until itoccupied about half the tube. The lead/bismuth alloy (45% lead, 55%bismuth) was introduced into the tube above the copper rod, and thesecond pre-tinned copper rod pushed into the unoccupied part of thetube. The tube was introduced into the vertical furnace, heated to 150°C. and the rods pushed together. The tube containing the rods wasmaintained at 150° C. for 5 minutes and then taken out and allowed tocool.

What we claim is:
 1. A conductor comprising a tubular sheath oftitanium, zirconium or tantalum or an alloy of one of these metals, saidsheath having at least one core element of another metal insertedtherein and bonded thereto and having a coating of tinning metal oralloy thereon.
 2. A conductor as in claim 1 wherein the core element iscopper.
 3. A conductor as in claim 1 wherein the core element isaluminum.
 4. A conductor as in claim 1 wherein the core element is ametal of greater electrical conductivity than the coated sheath.
 5. Aconductor as in claim 1 wherein the alloy constituting the sheath is analloy of titanium and zironium containing up to 14% by weight ofzirconium.
 6. A conductor as in claim 1 wherein the alloy constitutingthe sheath is an alloy of titanium and a platinum group metal containingup to 5% by weight of the platinum group metal.
 7. A conductor as inclaim 6 wherein the platinum group metal is selected from the groupconsisting of platinum, rhodium, iridium, ruthenium, osmium orpalladium.
 8. A conductor as in claim 1 wherein the alloy constitutingthe sheath is an alloy of titanium and niobium or tantalum containing upto 10% by weight of niobium or tantalum.
 9. A conductor as in claim 1wherein the coating is tin.
 10. A conductor as in claim 1 wherein thecoating is a binary alloy containing a major proportion of tin and aminor proportion of bismuth, cadmium or zinc.
 11. A conductor as inclaim 1 wherein the coating is a binary alloy containing a majorproportion of cadmium and a minor proportion of bismuth or zinc.
 12. Aconductor as in claim 1 wherein the coating is a binary alloy containinga major proportion of zinc and a minor proportion of bismuth, cadminum,lead or tin.
 13. A conductor as in claim 1 wherein the coating is aternary alloy of tin/zinc/lead containing a major proportion of tin andminor proportions of zinc and lead.
 14. A conductor as in claim 1wherein the coated sheath is soldered to the core element.
 15. Aconductor as in claim 14 wherein the solder is an alloy of lead andbismuth.
 16. A conductor as in claim 15 wherein the alloy contains morethan 50% by weight of bismuth.
 17. A conductor as in claim 15 whereinthe alloy contains 55% by weight of bismuth and 45% by weight of lead.